The proliferation of fibroblasts cultured from psoriatic and normal skin was compared following addition of clobetasol propionate to the culture medium. Proliferation was stimulated at 0.1 microgram/ml and progressively inhibited at 1-10 micrograms/ml. A differential sensitivity to the drug was demonstrated: the fibroblasts from involved psoriatic skin were more stimulated than normal fibroblasts at 0.1 microgram/ml and then more inhibited at 10 micrograms/ml. The fibroblasts from uninvolved psoriatic skin displayed intermediate responses. There was some cytotoxicity in cultures of the psoriatic cells at 10 micrograms/ml. The results demonstrate a further abnormality of fibroblasts from psoriatic skin.